Virtual Endoscopy

One of the important clinical applications of 3D visualization and volume rendering is virtual endoscopy [18,35,41,49,50, 53,61,62,72,73]. Virtual endoscopy has the advantage of noninvasive exploration of body cavities, which may be useful for screening procedures. Two approaches are used for

FIGURE 13 Segmentation and volume modeling of skull and selected contents from high-resolution cryosection images from Visible Human Female Dataset. In addition to skull, ocular orbits, muscles, and optic nerve can be visualized, as well as the semicircular canals, cochlea, and vestibule of the inner ear.

FIGURE 13 Segmentation and volume modeling of skull and selected contents from high-resolution cryosection images from Visible Human Female Dataset. In addition to skull, ocular orbits, muscles, and optic nerve can be visualized, as well as the semicircular canals, cochlea, and vestibule of the inner ear.

producing 3D computed endoscopic visualizations. The first is direct volume rendering, placing the perspective viewpoint within the volume of interest and casting divergent rays in any viewing direction [61], with application of any or all of the operating conditions used in conventional volume rendering. The other approach, illustrated in Fig. 16, is to produce volume models to obtain real-time exploration of the region of interest [53]. In this technique, a 3D volume scan, generally from CT or MRI, is segmented and modeled, as previously described. A common viewpoint is then located within the anatomic model, and a sequence of surface rendered views is produced from changing camera positions within the model. Figure 17 shows virtual endoscopic views computed within various body organs from the Visible Human Male Dataset [1]. Beginning at top right and proceeding clockwise, endoscopic views are computed and displayed within the skull looking down through the spinal column, inside of the trachea, inside of the heart, within one of the ureters, within the colon, in the spinal column, in the stomach and in the esophagus, respectively.

The high-resolution Visible Human Female Dataset [1] also has been segmented with many endoscopic views shown as Fig. 18. Segmentations of the right breast are shown superimposed on the body section at the top of Fig. 18 with glandular tissue and vessels rendered, including an angioscopic view within one of the mammary vessels. At the bottom of the figure are shown the segmented ovaries, fallopian tubes, uterus, and bladder, with endoscopic views in both the left and right fallopian tubes and in one of the ureters.

Texture mapping can lend significant realism to rendered views of segmented anatomic objects [32]. Figure 19 shows segmentations of the lungs and airways from the Visible Human Female Dataset [1] with RGB color values from the cryosection data mapped pixel for pixel onto the segmented external and internal surfaces of the lung and the trachea. The textured luminal surface inside of the trachea in the pullout endoscopic view shows the endotracheal surfaces looking inferiorly toward the carina. Figure 20 shows endoscopic views within the colon computed from a spiral CT scan of a patient with polyps. A polyp in the sigmoid rising out of the luminal surface can be detected and enhanced, as shown in the upper right panel, and further segmented to reveal its vascular components. The geometry and composition of the polyp can be measured from the correlated raw CT data to help determine metastatic potential of the polyp and thus help guide therapy decisions.

One of the disadvantages of virtual endoscopy is that CT and MRI scans do not capture the fine detail in the mucosal lining of anatomic structures such as the airways or colon that can be

seen with a real light endoscope. Small blood vessels and color of inflamed regions are often important diagnostic features that are not captured by medical scanners. Artificial texture mapping can be used to enhance realism in the computed endoscopic views, but this is not patient-specific and can be misleading. However, precise location, size, and shape of abnormal structures such as lesions and masses can be accurately visualized with virtual endoscopy from any orientation, both within and outside of the region of interest, capabilities not possible with conventional endoscopy; and these can be obtained without inserting an uncomfortable probe into the body. This is the promise and rationale for virtual endoscopy as a screening procedure.